Cooking devices in professional and large-scale kitchens have a cooking chamber in which food is cooked. Normally this is done by means of heating elements and fans to generate a cooking chamber atmosphere as homogeneous as possible to which the food is exposed.
In addition, certain applications provide for electrical power to be available in the cooking chamber itself in order to cook the food or monitor the cooking process. In a first approach, a corresponding cable is laid into the cooking chamber to provide electrical power. However, the problem in this case is that connector plugs are required inside the cooking chamber to enable a removable cooking accessory such as a contact heating plate to be connected. The plug connector would have to transfer high power, entailing further problems concerning electrical insulation, particularly under the conditions prevailing inside the cooking chamber.
A further problem is that the wall of the cooking chamber has to be thermally insulated in the case of alternative cable ducting in order, on the one hand, to prevent the cooking chamber atmosphere from escaping and, on the other hand, to prevent the temperature in an adjacent electrical installation space accommodating the electronic components of a cooking device from rising too much. However, thermal insulation is very expensive, resulting in an increase in the expense of installation and the associated manufacturing cost.
For this reason, in the prior art, power transfer units are provided in which the electrical energy is to be transferred into the cooking chamber cordlessly or wirelessly. To this end, resonant power transfer units comprising a transmitter and a receiver are normally used. The transmitter and the receiver each have at least one free oscillating circuit which includes a corresponding transmitting or receiving coil.
Between the transmitter and the receiver there is normally the wall of the cooking chamber which consists of stainless steel. The electrically conductive cooking chamber wall prevents efficient wireless energy transfer because high power can only be transferred at high frequencies by means of resonant coupling. The frequencies normally used in power transfer are above 10 kHz. By contrast, the oscillating circuits for low frequencies would be unmanageably large. The corresponding resonant frequencies of the respective coils are matched to one another. Otherwise a poor level of efficiency ensues, such that wireless energy transfer is either impossible or is only possible to a limited extent. In fact, induced shielding currents cause electrical losses to increase quadratically with the frequency, for which reason in the case of typical transfer frequencies the major proportion of the power remains in the cooking chamber wall.
In addition, non-resonant magnetic power transfer requires a good magnetic coupling between source and receiver coils. This is possible in the case of a transformer when both coils are placed on a common closed, highly permeable core. However, in the case of a cooking device this core is interrupted because the cooking chamber wall is not designed to be opened. The cooking chamber wall thus forms a gap at two places between the two halves of the core, this gap comprising the non-magnetic material—in other words, the metal sheet comprising the cooking chamber wall. However, such gaps reduce the coupling between the coils considerably. This increases the losses and larger coils are required in order to transfer the same amount of power.
The object of the invention is to provide a simply constructed transfer of wireless energy in the case of a cooking device by simple means and cost-effectively.